1. Field of the Invention
The present invention relates to an optical diffusion device adapted for controlling diffusion properties, and also relates to a projection screen, a design member and a security medium, which use the optical diffusion device, respectively.
2. Background Art
As a conventional optical diffusion device composed of a liquid crystal polymer, an optical compensation device, an optical reflection device and an optical phase difference device, respectively using a nematic liquid crystal, a cholesteric liquid crystal, a discotic liquid crystal and the like, have been known. Such devices are generally utilized as the optical diffusion devices, such as liquid crystal panel displays, security members, optical measuring devices, optical parts, liquid crystal projectors, rear-projection televisions, projection screens and the like.
Of these devices, the optical reflection device is representative of the optical diffusion devices with a structure of cholesteric orientation. Specifically, such an optical diffusion device is utilized as a polarized-light separating reflection device in a liquid crystal display member or as a color filter, or otherwise utilized as a negative C-plate optical compensation device or the like.
As the projection screen, a polarizing screen using the cholesteric liquid crystal has been known. In recent years, screens and reflectors for the LCD, using a cholesteric liquid crystal polymer layer configured for controlling the diffusion properties, have been known.
In the optical diffusion device using such a cholesteric liquid crystal polymer layer, a method for improving luminance distribution and viewing angle properties has been known, in which the cholesteric liquid crystal polymer layers, respectively having different viewing angle properties, are arranged in a thickness direction (e.g., see Patent Document 1).    Patent Document 1: JP 2005-107296 (TOKUKAI No. 2005-107296)
However, in such construction having the cholesteric liquid crystal polymer layers laminated therein, it is quite difficult to obtain the optical diffusion device that can exhibit discontinuously different optical properties only in desired regions, as seen in patterning, without creating any difference in grades and/or breaks.
Additionally, in such a prior art construction, each laminated cholesteric liquid crystal polymer layer is configured for diffusing and reflecting light of the same wavelength range. Thus, the remaining light other than the light reflected by the cholesteric liquid crystal polymer layer located on a front side will be reflected by the cholesteric liquid crystal polymer layer located on a rear side. Therefore, optical diffusion due to each cholesteric liquid crystal polymer layer depends on the film thickness of the cholesteric liquid crystal polymer layer. This makes it difficult to adequately control the optical diffusion properties over the entire system. Furthermore, in some cases, a part of incident light is likely to be repeatedly reflected between the cholesteric liquid crystal polymer layer located on the front side and the cholesteric liquid crystal polymer layer located on the rear side, leading to occurrence of a stray light.